The present invention relates to a zero crossing detection circuit for detecting reversal of polarity of a voltage applied to a telephone line or the like.
In the related art, the following publications disclose examples of technology in this field.    Publication 1: Japanese Patent Laid-open No. Hei. 3-124154    Publication 2: Japanese Patent Laid-open No. Hei. 10-75323
FIG. 2(a) and FIG. 2(b) are circuit diagrams showing an example structure of a polarity reversal detection circuit of the related art. These circuits are for non-ringing terminals such as telemeters and detect polarity reversal information such as seize signals or restore signals transmitted from a telephone through a telephone line.
FIG. 2(a) is the polarity reversal detection circuit of publication 1, having a resistor R1, capacitor C1 and photocoupler PC connected in series across telephone lines L1 and L2. The photocoupler PC is configured of two light emitting diodes connected in opposite polarity in parallel with each other at the input side, and a light signal couples between the input side and a phototransistor PT at the output side. One end of the output side of the photocoupler PC is connected through a resistor R2 to a source voltage VCC, and the other end is connected to ground potential GND. A detection signal DET is then output from the one end of the output side of the photocoupler PC.
With this polarity reversal detection circuit, when a voltage across the telephone lines L1 and L2 is fixed, the capacitor C1 is charged by this voltage and current does not flow in the input side of the photocoupler PC. Accordingly, the phototransistor PT of the photocoupler PC is in an OFF state and the detection signal DET becomes the source voltage VCC. Here, if there is a reversal in the polarity of the voltage applied across the telephone lines L1 and L2, current flows in the input side of the photocoupler PC until the capacitor C1 is charged to the reverse polarity voltage. Then, depending on the direction of current flow, one of the light emitting diodes D1 and D2 of the photocoupler PC emits light and the phototransistor PT is temporarily put into an ON state. In this way, a detection signal DET at substantially the ground potential GND is output in a pulse shape.
FIG. 2(b) is the polarity reversal detection circuit of publication 2, comprised of a full wave rectifier circuit REC connected across telephone lines L1 and L2, and edge detection circuits EDG1 and EDG2 connected across the telephone lines L1 and L2 and a negative electrode GND side of the full wave rectifier circuit REC.
The edge detection circuits EDG1 and EDG2 have the same structure, and, for example, edge detection circuit EDG1 comprises a current mirror M1 comprised of transistors T1 and T2, a current mirror M2 comprised of transistors T3 and T4, a switched current amplifier S1 comprised of transistors T5 and T6 and resistor R1, a constant current amplifier C1 comprised of transistors T7 and T8 and resistor R2, and Zener diodes Z1 and Z2.
Emitters of transistors T1 and T2 of the current mirror M1 are connected to the telephone line L1, and the bases of these transistors T1 and T2 are connected to the collector of transistor T2 and emitters of transistors T3 and T4 of the current mirror M2.
The cathode of Zener diode Z2 is connected to the collector of transistor T1, while the anode of the Zener diode Z2 is connected to the collector of transistor T5 of the current amplifier S1 and to the bases of transistors T5 and T6. The emitter of transistor T6 is connected to the negative electrode GND, while the emitter of transistor T5 is connected to the negative electrode GND of the full wave rectifier circuit through resistor R1.
The bases of transistors T3 and T4 of current mirror M2 are connected to the collector of transistor T4 and the collector of transistor T8 of the current amplifier C1.
The collector of transistor T3 is connected to the cathode of Zener diode Z1, while the anode of Zener diode Z1 is connected to the collector of transistor T6 of the current amplifier S1, the collector and base of transistor T7 of current amplifier C1, and the base of transistor T8.
The emitter of transistor T7 is connected to the negative electrode GND of the full wave rectifier circuit REC, while the emitter of transistor T8 is connected to the negative electrode GND through resistor R2. A detection signal DET1 is then output from the emitter of transistor T8.
Here, a breakdown voltage Voff of the Zener diode Z2 is set so as to be higher than a breakdown voltage Von of the Zener diode Z1, and lower than a voltage Vp applied to the telephone line L1. Also, a loop current amplification factor of a fixed current loop CL looping the current amplifier C1 from the current mirror M1 is set so as to become 1 when the Zener diode Z1 is in an on state.
In this polarity reversal detection circuit, polarity of a voltage applied to the telephone lines L1 and L2 reverses and the potential V1 of the telephone line L1 rises from zero.
When the potential V1 is lower than the breakdown voltage Von of the Zener diode Z1, both of the Zener diodes Z1 and Z2 are in an off state, and so current does not flow in the edge detection circuit EDG1.
If the potential V1 exceeds the breakdown voltage Von, the Zener diode Z1 turns on and a fixed current starts to flow in the fixed current loop CL.
If the potential V1 rises further and exceeds the breakdown voltage Voff, the Zener diode Z2 is turned on and current starts to flow in transistor T5 of current amplifier S1. As a result, a corresponding current also flows in transistor T6. Since current flowing in transistor T6 flows from transistor T3 of current mirror M2 through Zener diode Z1, the loop current amplification factor of the fixed current loop CL becomes smaller than 1. Therefore, current flowing in the fixed current loop CL becomes gradually smaller, and an off state finally results. After that, even if the potential V1 rises even further, current will not flow in the fixed current loop CL. As a result, when the potential V1 of the telephone line L1 rises up, a fixed current only flows in transistor T8 for a short time that elapses from voltage Von to Voff, and a detection signal DET1 having a pulse shape depending on the fixed current is output.
However, with the polarity reversal detection circuit of the related art, there are the following problems.
Specifically, the polarity reversal detection circuit of FIG. 2(a) is characterized by an extremely simple circuit structure, but a capacitor C1 having a large capacitance of, for example, 1 μF and a high withstand voltage of 250V or more is required, making it impossible to incorporate in an integrated circuit.
On the other hand, the polarity reversal detection circuit of FIG. 2(b) is normally held in an off state when the potential of the telephone lines L1 and L2 is constant. Therefore, transistors T1, T3 and T8 are required to have a high withstand voltage of, for example, at least 250V so as to withstand the voltage across the telephone lines L1 and L2. Also, this circuit has many other circuit structural elements besides the high withstand voltage transistors, which means that the size of an integrated circuit would be large.